This is a poetic form, found in Homer, in tragic verse, and in Theocritus. See LSJ s.v. φέρτατος. There's one conversational instance of it listed in Plato's Phaedrus, something like, "my very good man."

I should guess that for colloquial Koine one ought really to read carefully through Epictetus and draw out expressions of address, interrogation, response and evaluative comments. It might be worth doing a comparative study of expressions in Platonic dialogue and the dialogues of Epictetus as set down by Arrian.

James, a servant of God and of the Lord Jesus Christ, To the twelve tribes in the Dispersion:
Greetings. Count it all joy, my brothers, when you meet trials of various kinds, for you know that the testing of your faith produces steadfastness. And let steadfastness have its full effect, that you may be perfect and complete, lacking in nothing.